City power used by people mostly has a voltage range between 110V and 220V. However, most electronic products require a voltage lower than the aforesaid range. Hence a transformer usually is needed to adjust the voltage of the power source supplied to the electronic products. A conventional transformer generally comprises a coil rack, an induction coil and a magnetic core coupled together. The induction coil usually is formed by winding a cylindrical metal wire in a helical shape and coated with a layer of insulation lacquer on the surface thereof. For instance, R.O.C. patent No. I305654 discloses a technique including a first coil to receive input signals, a second coil to transmit the signals input to the first coil, a first column wound by the first coil and a first magnetic core surrounding a first wall of the first column, and a second column wound by the second coil and a second magnetic core surrounding a second wall of the second column. The first magnetic core has relative permeability greater than that of the second magnetic core. The first column opposes and connects to the second column to allow the first wall also to oppose and connect to the second wall. The first coil or second coil generates a magnetic loop with magnetic flux to pass through the first column and second column.
R.O.C. publication No. 200923985 discloses a high voltage transformer with adjustable magnetic flux leakage. It comprises a winding rack unit, a primary coil, a first secondary coil and a magnetic core unit. The primary coil is wound on the winding rack unit. The first secondary coil also is wound on the winding rack unit. The magnetic core unit includes a first magnetic core running through the winding rack unit and a second magnetic core running through the winding rack unit to connect to the first magnetic core. The second magnetic core has a second base column, two first and second secondary side columns extended from two opposite ends of the second base column and extended towards the first magnetic core, and a magnetic regulation end extended from the second base column to the first magnetic core and located between the secondary side columns. The magnetic cores form a magnetic flux path connecting the first secondary coil and primary coil. By regulating the magnetic flux leakage via the magnetic regulation end, impedance match can be achieved.
All the aforesaid transformers have a coil rack for winding the coils. The coil rack is formed at a definite volume which cannot be shrunk as desired. Nowadays electronic products are more complex and the demand of slim and light is increasing. Requirement for shrinking transformer size also has been raised constantly. The existence of the coil rack hinders miniaturization possibility of the electronic products.